In automatic copying and printing with electrostatic or xerographic techniques, where printing of an image on both sides of the print media sheet has been required, it is necessary to invert the sheet by reversing the sheet with respect to the leading and trailing edge in the direction of feed after printing on one side. This results in what was originally the leading edge with respect to the direction of feed now becoming the trailing edge. Typically such copying machines register against the leading edge of the sheet for positioning the sheet during printing of the first side. Inverting the sheet to present an opposite edge as the leading edge for registration introduces error in locating the image on the second side of the sheet with respect to the image printed on the first side. Where the pages are to be bound or placed in a notebook the misorientation of the printed image on opposite sides is quite noticeable to the reader upon turning the pages and is considered to be unacceptable printing quality.
In an office or enterprise printing arrangement, duplex printing may be accomplished by employing two common printing engines, with the output of the first, after being inverted, feeding into the second identical printing engine. This type arrangement permits serial duplex printing or parallel dual simplex printing by an operator separately feeding sheet stacks to each printing engine individually. However, this does not permit continuity in scheduling print jobs. Thus, it has been desired to provide a way or means of enabling automatic electrostatic duplex printing in a manner which eliminates the registration errors associated with positioning the image on the opposite side with respect to opposite edges of the print sheet media. It has further been desired to permit scheduling duplex and simplex printing continuously on equipment utilizing plural identical printing engines.